The invention relates to interdigital filters, and especially to duplexers including multiple interdigital filters within a single frame, functioning as duplexers.
Interdigital filters are well-known to those skilled in the art of microwave frequency apparatus, and are described in "Interdigital Band-Pass Filters", by G. L. Matthaei, IRE Transactions on Microwave Theory Techniques, November, 1962, page 479 and also in the text "Microwave Filter, Impedance-Matching Networks and Coupling Structures", by G. Matthaei, L. Young, and E. M. T. Jones, 1980, published by Artech House, Inc. Interdigital filters include a series of spaced, parallel conductive quarter wavelength resonators in a rectangular conductive housing and arranged in an interdigitated fashion in the sense that opposite ends of adjacent resonators are electrically grounded to the housing. The center frequency of an interdigital band-pass filter is determined by the lengths of its resonators. The interdigital filter bandwidth is determined by the spacing between adjacent resonators, and the width of each resonator determines its impedance. The number of resonators determines the selectivity of the interdigital filter, i.e., the steepness of the "skirt" of its band-pass characteristic.
Duplexers are widely used to couple transmitters and receivers to a common antenna. Multiple cavity interdigital filters also are known. U.S. Pat. No. 3,597,709 discloses a structure in which two separate interdigital filters are joined by a common wall having apertures therein to allow coupling of rf energy between the two cavities. U.S. Pat. No. 3,818,389 discloses an interdigital filter structure in which two cavities bounded by the same parallel face plates share a common output resonator. However, the cavities are disposed in end-to-end relationship, with the common resonator being located between them. This structure would not be practical where high selectivity and minimum physical length of the structure is needed. Neither of the foregoing dual cavity interdigital filter structures solve the problems associated with making a minimum size duplexer with interdigital filter structures.
Although duplexers such as the one shown in FIG. 5 have been constructed using interdigital filters, wherein a transmitter 91 and a receiver 96 are coupled to a common antenna 101, it is necessary to very precisely cut the lengths of cables 94 and 99, which couple interdigital filters 93 and 98, respectively, to a T-connector 95 that is connected to the antenna cable 100.
There is an unmet need for a practical interdigital filter duplexer structure that occupies minimum front panel space in an equipment rack and avoids the need to provide precisely cut lengths of cable to connect the "transmitter" filter and "receiver" filter of a duplexer to the common antenna.